If no such debate occurs, this market will resolve to no. If the person is cut off, but it is very clear what word they were in the middle of saying, that will be sufficient to resolve to yes. It will resolve yes no matter who says the word (Trump, Biden, or a moderator). Made up words will not count. If a candidate simply says "Z" that will not count, as "Z" is not a word.

Any word in the following list will count:

https://www.thefreedictionary.com/words-that-start-with-z

No other words will count.

EDIT:

This debate is planned to have no audience, but if someone breaks in and shouts the word "Zebra" or something, that will count. Hacking a speaker to say "Zebra" would not though. The word needs to be spoken by a physically present human in the same room as the debaters and moderator (so someone standing outside of the room yelling through a megaphone loud enough to be heard through walls would not count.)